In this section, we will put forward some key ideas that will operate throughout these web pages; indeed, throughout the study of what we call Geometric Optics.
HOW WE SEE THINGS:
In order for us to "see" an object, light must travel from that object to our eyes. There are two ways that light can do that: 1) The light can be emitted by an object like the filament of a light bulb. 2) Light can reflect off of an object like the words on the page of a book.
Our eyes form an image from the light that is received onto the retina, where the rods and cones are stimulated. Our brains interpret the signals coming from the eyes and at this point we can say that we actually "see" the object.
WHERE DOES LIGHT COME FROM:
Light originates in electron or molecular transitions. Therefore, the originating place for light is essentially a point. A given light source, however, contains a very large number of these point sources.
An instructive way to look at a light source is to visualize it as a series of sources, each one quite small in size, each one emitting light. In addition to being a large number of small points, each emitting light, the light from each point travels out in all directions. This combination of characteristics is important to keep in mind as we observe optical phenomena.
WHAT KIND OF LIGHT IS REFLECTED LIGHT?
Reflected light behaves much like emitted light, with each point on the object acting like a point source. Light strikes the object and reflects off, often in all directions. Such a reflection is called diffuse. Highly polished objects, though, only reflect light at the angle it strikes. We will study these under the unit called mirrors.
WHAT IS LIGHT LIKE?
Most of the light we encounter in our daily lives is diverging. All of the images we see started out with light that diverged from a series of point sources, either from a light emitter or from a reflector.
Light is emitted in all directions from a given point source. This is called diverging, another name for spreading out. One issue that we deal with is how rapidly light is diverging when it gets to us. If the object is close to us, the angle of divergence is still fairly large. As the object's distance away increases, the angle decreases. At the extreme, astronomical distances, the distance away is so great that the light rays are essentially parallel when they get to our eyes.
CAN WE GET ANY OTHER KIND OF LIGHT?
Under some special circumstances, we can get light that is essentially a series of parallel rays. This is not the most common type of light, but can be found in high-quality lasers, for example.
In even rarer circumstances, we find light rays that are converging, or coming together rather than moving apart. In general, we won't deal with this type of light due to its scarcity.
WHAT IS THE MODEL FOR LIGHT?
The model that we use on this pages is called the ray model. Light is pictured as a series of rays, traveling at very high speeds in straight lines from points on the source or on the object. It is the behavior of light "rays" that we will study. If time permits we will investigate other models for light.
DO RAYS ALWAYS TRAVEL IN STRAIGHT LINES?
As we will see, rays travel in straight lines until they interact with things like mirrors, or when they enter a new material or medium. This is taken up in the section on refraction.
In general, light travels in a straight line as long as it is in a single medium such as air, water, glass, etc.
SUMMARY:Think of light as a series of rays, starting from point sources, diverging out into the world.
Ask what happens when this type of light interacts with optical devices like mirrors, prisms, lenses, etc. Carefully conduct experiments and draw your conclusions.
When you see an image, ask yourself how the light rays must have travelled in order to form that image. Ask the question, "Where did the light rays come from or where do they seem like they came from?" (Even if grammatically it is a bit incorrect.)
As you search for the answers to these questions, you'll be doing Physics! GOOD LUCK!